Galactic globular clusters constitute an important rung on the extragalactic distance ladder. They are nearby enough to be resolved into individual stars, making them ideal
objects to calibrate the brightnesses of physically interesting stars like RR Lyrae or Type II Cepheids. In addition they are also massive enough to contain statistically significant
samples of these stars. Hence globular clusters are useful to determine the slopes and zero-points of RR Lyrae period-luminosity (P/L) relations [1]}, [2]}
as well as a possible metallicity dependence of the zero points [3]}. Globular clusters can also be used as calibrators for other distance methods, like the tip of the red giant branch distance method (TRGB), which
allows to determine distances to external galaxies without having to rely on P/L relations of variable stars [4]}, [5]}, [6]}.
